A conventional automatic transmission device for a vehicle has a shift range changing device and a parking condition setting device, wherein a shift range (including a parking condition) is manually changed by a vehicle driver. Recently, a shift range control apparatus, such as disclosed in Japanese PCT Patent Publication No. 2004-513307, has been brought into a market, according to which a shift range changing device and a parking condition setting device are operated by an SBW actuator.
In the above SBW actuator of the shift range control apparatus, rotational force is produced by an electric motor. Accordingly, if any failure for current supply occurred, in which current supply from a vehicle battery to the electric motor is cut off due to any reason, the SBW actuator can no longer operate.
It may be possible to provide an additional battery (e.g. a capacitor) to operate the electric motor of the SBW actuator, just in case that the failure of the current supply from the vehicle main battery occurred.
In the case, however, that such an additional battery (e.g. the capacitor) is provided to operate the electric motor of the SBW actuator and to thereby drive the shift range changing device and the parking condition setting device, a large capacity is necessary for the additional battery (capacitor). Namely, the capacitor having the large capacity must be mounted on a vehicle, which would cause an increase of cost.